[Steuard]

There is a "Customize" option in the installer that seems to provide an option to not install MacGPG, but I haven't tried that to make sure.

[cel]

Yes, you can choose to install GPGMail, GPGPreferences, etc., without MacGPG, and use your own gpg binaries such as from homebrew.

[gcv]

I tried installing GPGMail stand-alone with a previous version, and it caused Mail.app to crash on start, presumably because /usr/local/bin/gpg was not available.